Erosion and corrosion resistant components and methods thereof

ABSTRACT

A coating system and a method of applying the coating system on an article. The coating system includes a sacrificial coating on a surface of the article and an erosion-resistant coating on the sacrificial coating, wherein the erosion-resistant coating comprises a layer of a polymeric material. The sacrificial coating is more anodic than the surface or the erosion-resistant coating.

BACKGROUND OF THE INVENTION

The present invention generally relates to coatings capable of exhibiting erosion and/or corrosion resistance. More particularly, this invention relates to coatings suitable for use on airfoil components, including compressor airfoils of gas turbines, to promote the corrosion and erosion resistance thereof and to methods of manufacturing airfoil components with such coatings.

Stainless steel blades, such as those used in the compressors of land-based gas turbine engines used in industrial applications (for example, power generation), have shown susceptibility to erosion and corrosion pitting of their airfoil surfaces that are believed to be associated with various electrochemical dissolution processes enabled by the impingement of water droplets and chemical species present in the droplets, intake air, and combinations thereof. Electrochemically-induced corrosion and erosion phenomena occurring at the airfoil surfaces can in turn result in cracking of the components due to the cyclic thermal and operating stresses experienced by these components. Water droplet exposure can result from use of on-line water washing, fogging and evaporative cooling, or various combinations of these processes that may be employed to enhance compressor efficiency. Water droplet exposure can also result from the environments in which land-based gas turbines operate, for example, highly corrosive environments such as those near chemical or petrochemical plants where various chemical species may be found in the intake air, or those at or near ocean coastlines or other saltwater environments where various sea salts may be present in the intake air, or combinations of the above, or in other applications where the inlet air contains corrosive chemical species.

Though improvements in erosion and/or corrosion resistance can be achieved through the use of different materials, such as nickel-base or titanium-base alloys, this approach may not solve water droplet erosion or corrosion pitting problems since these materials may also have susceptibility to the associated electrochemical processes. Other potential drawbacks to the use of materials other than stainless steels include higher costs of their alloy constituents, the need for redesign of the blades, including airfoil surfaces, due to the different metallurgical and mechanical properties of the materials, and issues relating to overall robustness of the blades resulting from potential sensitivity to other degradation phenomena, such as various rub and fretting wear mechanisms.

Corrosion-resistant airfoil coatings and methods of making steel airfoil components with corrosion-resistant coatings are described in U.S. Pat. Nos. 5,098,797 and 5,260,099 to Haskell. These patents describe a corrosion-resistant coating that includes a sacrificial undercoat of a metal that is above iron in the electromotive series, and a ceramic overcoat. The ceramic material is applied at a temperature of 600° F. or less to avoid reduction of fatigue resistance of the alloy used to form the component, which is disclosed as a stainless steel alloy.

While approaches of the type disclosed in the Haskell patents are capable of improving the erosion or corrosion resistance of stainless steel compressor blades, additional approaches for achieving further improvements would be desirable.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a method suitable for depositing a coating system on an article, such as a compressor blade of a land-based gas turbine, and a coating system deposited thereby that provides improved corrosion and erosion resistance to the article.

According to a first aspect of the invention, a coating system for an article includes a sacrificial coating on a surface of the article and an erosion-resistant coating on the sacrificial coating. The erosion-resistant coating comprises a layer of a polymeric material, and the sacrificial coating is more anodic than both the surface and the erosion-resistant coating.

According to a second aspect of the invention, a method of applying a coating system on an article includes depositing a sacrificial coating on a surface of the article and depositing an erosion-resistant coating on the sacrificial coating, wherein the erosion-resistant coating comprises a layer of a polymeric material. The sacrificial coating is more anodic than both the surface of the article and the erosion-resistant coating.

A technical effect of the invention is the ability to improve the corrosion and erosion resistance of articles that are subjected to erosion and/or corrosion, including compressor blades that are susceptible to water droplet erosion and corrosion pitting. In particular, it is believed that, by providing such an article with a sacrificial coating that is more anodic than the surface of the article and providing an erosion-resistant coating thereon formed of a polymeric material, the corrosion and erosion resistance of the article can be significantly improved.

Other aspects and advantages of this invention will be better appreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic perspective view of a compressor blade of a type used in gas turbine engines and suitable for protection with coatings in accordance with an aspect of the invention.

FIG. 2 is a cross-sectional view of section 2-2 of FIG. 1.

FIG. 3 is a schematic cross-sectional view of an embodiment of region 90 of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described below in reference to airfoil components whose airfoil surfaces are protected by a coating system that includes a sacrificial coating and an erosion-resistant coating. The coating system is particularly useful for use within the compressor sections of land-based gas turbine engines, including components such as rotating compressor blades and stationary compressor vanes, shrouds, and other surfaces within the compressor section. The coating system may also be applicable to other components that may be subject to water droplet erosion and/or pitting and crevice corrosion, nonlimiting examples of which include vanes, nozzles and shrouds within the turbine section of a gas turbine engine, combustor liners, diaphragm components, seal components, valve stems, nozzle boxes, nozzle plates, and the like as well as components in steam turbines.

FIG. 1 schematically represents a compressor blade 1 of a type that may be used in land-based gas turbine engines. The blade 1 comprises an airfoil 10 whose surfaces are desired to exhibit improved resistance to particle impact, water droplet erosion and crevice corrosion. The blade 1 is represented as configured to be removably attached to a central hub or disk, though the blade 1 could alternatively be an integral portion of a hub or disk, in which case the combination of the hub/disk and its blades is commonly referred to as a blisk. However, while the embodiments herein are illustrated with reference to the particular compressor blade 1 represented in FIG. 1, they are broadly applicable to a wide variety of other gas turbine engine components, as previously noted.

Referring to FIG. 1, the blade 1 is illustrated as having, in addition to the airfoil 10, a leading edge 14, a trailing edge 18, a tip edge 22 and a blade root 26. The span 28 of the airfoil 10 extends from the tip edge 22 to the blade root 26. The surface of the airfoil 10 comprehended within the span 28 constitutes an airfoil surface 32 of the blade 1, and is therefore exposed to the flow path of air entering the gas turbine engine at its inlet (not shown) upstream of the compressor section.

FIG. 2 shows the airfoil surface 32 of the airfoil 10 as comprising convex and concave surfaces 30 and 34 that extend between the leading edge 14 and trailing edge 18. The dashed line 38 extending from the leading edge 14 to the trailing edge 18 defines the width or chord of the airfoil 10. The double-headed arrow 42 between the convex and concave surfaces 30 and 34 defines the thickness (usually measured as the “maximum” thickness) of the airfoil 10.

The greatest erosion and corrosion damage to the airfoil surface 32 tends to occur at a leading edge section 46 of the airfoil 10, particularly with regard to the initiation of erosion or corrosion pitting, and especially at or proximate to the leading edge 14. Referring to FIGS. 1 and 2, the area of greatest erosion damage tends to occur in a tip edge portion 50 of the airfoil 10 that defines the tip edge 22, and especially at or proximate to the tip edge 22, and also tends to be focused in a portion 54 of the concave surface 34 represented in FIG. 2 as directly forward of the trailing edge 18 and to a lesser extent in a portion 58 of the concave surface 34 represented in FIG. 2 as directly aft of the leading edge 14. FIG. 2 represents a coating system 62 of a type noted above as comprising sacrificial and erosion-resistant coatings, described in more detail below. The coating system 62 may be disposed over all or any portion of the airfoil surface 32, but is particularly suited for disposition on the portions 50, 54, and 58 of the airfoil surface 32 that are most susceptible to corrosion and erosion, as described above.

The blade 1 is represented in FIG. 2 as having a substrate 60, which may be made from various stainless steels and superalloys including, but not limited to, Fe-based, Co-based and Ni-based superalloy compositions. The superalloys may include aluminum and/or titanium as solutes. Generally, the aluminum and/or titanium concentrations are low (e.g., less than or equal to about 15 weight percent (wt %) each). Other optional components include chromium, molybdenum, cobalt (in Fe-based or Ni-based superalloys), tungsten, nickel (in Fe-based or Co-based superalloys), rhenium, iron (in Co-based or Ni-based superalloys), tantalum, vanadium, hafnium, columbium, ruthenium, zirconium, boron, yttrium, and carbon, each of which may independently be present in an amount of less than or equal to about 15 wt %. The substrate 60 of the blade 1 may be made from various grades of stainless steel, including, but not limited to, both 300 series and 400 series stainless steels. For example, the blade 1 may comprise type 450 stainless steel, a martensitic, age-hardenable alloy having a reported composition of, by weight, up to 0.05% carbon, up to 1.00% manganese, up to 0.030% phosphorous, up to 0.030% sulfur, up to 1.00% silicon, 14.00 to 16.00% chromium, 5.00 to 7.00% nickel, 0.50 to 1.00% molybdenum, 1.25 to 1.75% copper, niobium (columbium) in a minimum amount of 8 times that of the percent of carbon, and the balance (approximately 72.14 to 77.14%) iron and impurities.

The airfoil surface 32 is an outermost surface of the substrate 60 of the airfoil 10 represented in FIG. 2. FIG. 3 represents the coating system 62 as including a sacrificial coating 64 disposed on the substrate 60 and an erosion-resistant coating 66 disposed on the sacrificial coating 64. Hence, the structure may be described generally as a blade 1 having the airfoil surface 32, sacrificial coating 64, and erosion-resistant coating 66, wherein the sacrificial coating 64 is disposed on the airfoil surface 32 and the erosion-resistant coating 66 is directly disposed on the sacrificial coating 64. The coating system 62 may have any thickness that is effective for providing a predetermined amount of corrosion-resistance and erosion-resistance, including the sum of those described below for the sacrificial coatings 64 and the erosion-resistant coatings 66. The coating system 62 preferably provides suitable corrosion-resistance, particularly with regard to galvanic and crevice corrosion, and erosion resistance, particularly with regard to water droplet erosion, to the airfoil 10 at least in the portions 54 and 58 of the concave surface 34, more preferably over the entire or substantially the entire area of the concave surface 34, and most preferably over the entire or substantially the entire area of both of the convex and concave surfaces 30 and 34.

Preferably, the sacrificial coating 64 is more anodic than both the substrate 60 and erosion-resistant coating 66. By more anodic, it is meant that the electromotive force (emf) or corrosion potential with respect to a standard thermodynamic reference potential of the sacrificial coating 64 is more negative than that of either the substrate 60 or erosion-resistant coating 66 in a corrosive (reactant) species to which the blade 1 is exposed. These species may be ingested together with water droplets from the external environment, or may mix with water droplets that are deliberately introduced. For example, these species may include various ionic species, including those comprising, Cl⁻, Br⁻, F⁻, S²⁻, and/or others. Together with the water droplets, these species are capable of participating in various electrochemical reactions and thereby causing electrochemical erosion and corrosion of the airfoil surface 32. By making the sacrificial coating 64 electrochemically more anodic than either the airfoil surface 32 or the erosion-resistant coating 66, these species most likely will preferentially attack the sacrificial coating 64 rather than the airfoil surface 32.

Reference herein to the sacrificial coating 64 being disposed on the airfoil surface 32 means that it is attached and tightly adherent to this surface, preferably by virtue of chemical or metallurgical bonding, such that it is able to undergo normal operating and thermal stresses without exhibiting spallation or other coating degradation processes. The airfoil surface 32 may be treated to produce a residual compressive surface stress in order to reduce the tendency of any cracks or pits (or other features that might tend to cause a stress riser at the surface) from propagating from the airfoil surface 32 into the interior of the airfoil 10. Residual compressive stresses may be imparted to the airfoil surface 32 by shot peening, laser peening or other treatments that also produce residual compressive stresses, or other methods. The coating system 62 may also be disposed so as to include residual compressive stresses, preferably compressive stresses that are greater than those of the airfoil surface 32, more preferably where the airfoil surface 32 includes residual compressive stresses. For example, the sacrificial coating 64 may be formed to have a residual compressive stress of about 3792 MPa.

The sacrificial coating 64 may comprise one or more layers comprising, for example, Al, Cr, Zn, Al-based alloys, Cr-based alloys, Zn-based alloys, and/or combinations thereof which are preferably more anodic than the airfoil surface 32 or the erosion-resistant coating 66. Alternatively, the sacrificial coating 64 may comprise various glasses, ceramics, polymers and composites, in any combination, that include the above-mentioned metallic materials. For example, the sacrificial coating 64 may comprise an Al particle polymer composite. These metallic materials may further be used in particulate or other forms in various paints and composite materials, including those comprising various polymeric materials, including metal particle pigmented paints, such as aluminum particle pigmented paints having an aluminum content of about 70% or more, by weight.

The sacrificial layer 64 may be disposed either on the airfoil surface 32 or over a previously deposited coating system (not shown), but is particularly suited to being directly disposed on the airfoil surface 32, as this arrangement places the anodic material in direct electrical contact with the airfoil surface 32, thereby improving the likelihood of anodic protection of the airfoil surface 32. The sacrificial layer 64 may be deposited as a thin film or thick film layer by any suitable application or deposition method, including, but not limited to, plating (electroplating and electroless plating), dipping, spraying, painting, chemical vapor deposition (CVD), or physical vapor deposition (PVD), such as EB-PVD, filtered arc deposition, and more preferably by sputtering. Suitable sputtering techniques include, but are not limited to, direct current diode sputtering, radio frequency sputtering, ion beam sputtering, reactive sputtering, magnetron sputtering and steered arc sputtering. The sacrificial coating 64 may include a single layer, or may be provided in multiple layers, including a sacrificial coating 64 that includes a plurality of different materials as sub-layers disposed in a contiguous fashion to form the sacrificial coating 64. In a single layer configuration, the sacrificial coating 64 may have any suitable thickness needed to provide anodic protection of the airfoil surface 32, including to obtain a predetermined or design service life. For example, the thickness of the sacrificial coating 64 in the form of a thick film, such as a metal particle/polymer matrix paint, may range from about 120 to 730 micrometers. The thickness of the sacrificial coating 64 deposited using a thin film deposition method preferably has a higher density than the thick film sacrificial coatings 64 and may have a thickness in the range of about 5 to 50 micrometers.

As an example of a multilayer configuration, the sacrificial coating 64 may include a conductive undercoat layer and an overcoat layer disposed thereon of an inorganic binder having a plurality of ceramic particles and conductive particles embedded therein, as described in U.S. Pat. Nos. 5,098,797 and 5,260,099. In particular, the conductive undercoat layer may include a continuous, relatively thin, sacrificial metal layer, such as a layer of a nickel cadmium alloy. Such a sacrificial metal layer may be electroplated to a thickness of about 5 to 10 micrometers, preferably about 7.6 micrometers. Alternately, the conductive undercoat layer may be provided by flame or plasma spraying techniques known in the art, or preferably by applying a metallic paint, such as an aluminum particle/polymer matrix paint. When using the metallic paint, the airfoil surface 32 may be initially prepared by grit blasting and then drying, heating to cure and then consolidating the metal powder in contact with the airfoil surface 32, such as consolidation by glass bead blasting. Generally, a single application will be sufficient to produce the conductive undercoat layer of the metallic paint having a thickness in the range described above. The overcoat layer is disposed on the conductive undercoat layer and includes an inorganic matrix binder having a plurality of ceramic particles and conductive particles embedded therein. For example, the inorganic matrix binder may include a phosphate chromate binder having a plurality of aluminum oxide and chromium oxide ceramic particles and aluminum metal particles embedded therein. The binder may also include cobalt and other metal or conductive particles. The overcoat layer may be made using the methods disclosed in U.S. Pat. No. 3,248,251. The amount of the embedded metal particles may be selected to make the overcoat layer more anodic than the airfoil surface 32 or the erosion-resistant layer 66. The overcoat layer may be deposited in any suitable thickness. For example, where the inorganic matrix binder is a phosphate chromate binder having aluminum oxide and chromium oxide ceramic particles and aluminum metal particles embedded therein, the thickness of the overcoat layer may be about 3 micrometers or more.

As noted above, the erosion-resistant coating 66 of the coating system 62 may be disposed on the outer surface of the sacrificial coating 64. The erosion-resistant coating 66 may be one or more layers comprising an impact-absorbent, erosion-resistant layer of a polymeric material that is more elastic than the ceramic-based erosion materials of the prior art. If the sacrificial coating 64 includes the undercoat and overcoat layers as described above, such as the phosphate chromate inorganic matrix binder, the overcoat layer may also provide some erosion-resistance to the coating system 62 due to the hardness and abrasion resistance of the embedded ceramic particles; however, the phosphate chromate ceramic overcoat layer alone is not suitable for use as the erosion-resistant layer 66, as disclosed herein, because it does not provide sufficient erosion-resistance owing to its porous morphology.

The polymeric material of the erosion-resistant coating 66 is preferably chosen for its ability to absorb and re-emit the energy of the erosive attack. Other preferred properties include materials that are sufficiently hydrophobic to shed water, and materials that are anti-fouling to limit the amount of water-washing required in a compressor application. Suitable polymeric compositions include Si-based polymers such as, but not limited to, siloxanes, silicon alkyds, and flurosilicones, C-based polymers such as modified bituminous materials, modified tar, epoxy-based materials, elastomers, for example, polybutadiene, neoprene, butyl rubber, and the like, and/or combinations thereof. This may also include an erosion-resistant coating 66 that has multiple erosion-resistant layers of the same or different erosion-resistant materials, including those that also include a primer material layer to promote adherence of the erosion-resistant coating 66 to the sacrificial coating 64 or to other layers of the erosion-resistant coating 66. Suitable primer compositions include most epoxies as well as other materials that promote the adherence of the layers of the erosion-resistant coating 66.

Any suitable thickness of a single layer and a multilayer erosion-resistant coating 66 may be used, so long as it is effective to provide increased erosion resistance to the airfoil surface 32. For example, if the erosion-resistant coating 66 is composed of a single layer of a siloxane-based material, an effective layer thickness includes a minimum thickness of about 25 to a maximum thickness of about 380 micrometers. Preferably the single layer thickness is in the range of about 50 to 125 micrometers. The minimum thickness will be that which is effective to provide erosion-resistance greater than that of the bare airfoil surface 32, which generally will be a layer thickness sufficient to insure complete coverage of the airfoil surface 32, including features such as film cooling holes, etc., while avoiding coating defects sometimes associated with thin layers, such as pinholes. The maximum thickness may be any suitable thickness, but is preferably a thickness that is effective to provide a desired service life to the airfoil surface 32 in a predetermined operating environment, while also maintaining a desired level of adherence or bond strength to the sacrificial coating 64 to which it is applied.

The erosion-resistant coating 66 having one or more polymeric layers may be disposed as a thin film or thick film layer by any suitable application or deposition method, including airless spraying, dipping, brushing, high-volume, low-pressure (HVLP) spraying, or other suitable means.

While the invention disclosed herein have been described above with particular reference to an embodiment of a compressor blade 1, the coating systems of the types described herein may also be applied to other airfoil surfaces that may be subject to water droplet erosion or crevice corrosion as described herein.

Therefore, while the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the following claims. 

1. A coating system for an article, the coating system comprising: a sacrificial coating deposited on a surface of the article; and an erosion-resistant coating deposited on the sacrificial coating comprising a layer of a polymeric material, wherein the sacrificial coating is more anodic than both the surface and the erosion-resistant coating.
 2. The coating system of claim 1, wherein the article comprises a stainless steel or a superalloy.
 3. The coating system of claim 1, wherein the sacrificial coating comprises a layer of Al, Cr, Zn, an Ni—Al alloy, an Al—Si alloy, an Al-based alloy, a Cr-based alloy or a Zn-based alloy, an Al polymer composite, or a combination thereof.
 4. The coating system of claim 1, wherein the sacrificial coating comprises a layer of a conductive undercoat and a layer of an overcoat disposed on the undercoat, the overcoat comprising an inorganic matrix binder having a plurality of ceramic particles and conductive particles embedded therein.
 5. The coating system of claim 1, wherein the polymeric material of the erosion-resistant coating comprises siloxanes, silicon alkyds, and flurosilicones, modified bituminous materials, modified tar, epoxy-based materials, elastomers, polybutadiene, neoprene, butyl rubber, or a combination thereof.
 6. The coating system of claim 1, further comprising a primer material layer between the sacrificial coating and the erosion-resistant coating.
 7. The coating system of claim 1, wherein the article is a compressor blade of a gas turbine.
 8. The coating system of claim 1, wherein the sacrificial coating has a thickness of about 5 to about 50 micrometers.
 9. The coating system of claim 1, wherein the erosion-resistant coating has a thickness of about 50 to about 125 micrometers.
 10. The article comprising the coating system of claim 1 thereon.
 11. A method of applying a coating system on an article, the method comprising: depositing a sacrificial coating on a surface of the article; and then depositing an erosion-resistant coating on the sacrificial coating, the erosion-resistant coating comprising a layer of a polymeric material, wherein the sacrificial coating is more anodic than the surface of the article or the erosion-resistant coating.
 12. The method of claim 11, wherein depositing the sacrificial coating produces a residual compressive stress in the sacrificial coating.
 13. The method of claim 11, wherein the surface of the article is defined by a substrate formed of a stainless steel or a superalloy.
 14. The method of claim 11, wherein the sacrificial coating comprises a layer of Al, Cr, Zn, an Ni—Al alloy, an Al—Si alloy, an Al-based alloy, a Cr-based alloy or a Zn-based alloy, an Al polymer composite, or a combination thereof.
 15. The method of claim 11, wherein the sacrificial coating comprises a layer of a conductive undercoat and a layer of an overcoat disposed on the undercoat, the overcoat comprising an inorganic matrix binder having a plurality of ceramic particles and conductive particles embedded therein.
 16. The method of claim 11, wherein the polymeric material of the erosion-resistant coating comprises siloxanes, silicon alkyds, and flurosilicones, modified bituminous materials, modified tar, epoxy-based materials, elastomers, polybutadiene, neoprene, butyl rubber, or a combination thereof.
 17. The method of claim 11, further comprising depositing a primer material layer between the sacrificial coating and the erosion-resistant coating prior to depositing the erosion-resistant coating.
 18. The method of claim 11, wherein the article is a compressor blade of a gas turbine.
 19. The method of claim 11, wherein the sacrificial coating has a thickness of about 5 to about 50 micrometers.
 20. The method of claim 11, wherein the erosion-resistant coating has a thickness of about 50 to about 125 micrometers. 